Stair-Climber

Chapter 1 

Stair-Climber 

Doug Carlson 

1

2 Chapter # • Chapter Title 

Bill of Materials 

These are the parts you will need to build the Stair-Climber as shown. 

Introduction 

Stair-Climber is the latest in a series of models based on the tri-star wheel concept.The 

tri-star wheel is a triangular star-wheel arrangement in which the wheels can be driven 

in a normal fashion for rolling over flat terrain (Figure 1.1), traversing a terrain gap 

(Figure 1.2) or climbing over obstacles, as the whole assembly can be rotates (Figure 1.3). 

One extreme obstacle for many robotic vehicles—but for which the tri-star configuration 

really excels—is climbing stairs. When was the last time you saw something with wheels 

actually climb a set of stairs?

Stair-Climber • Chapter 1 3 

Figure 1.1 Rolling Mode 

Figure 1.2 Traversing a Terrain Gap

4 Chapter # • Chapter Title 

Figure 1.3 Climbing Mode 

I came across this concept some years back on Cynde Callera’s LEGO Web page: 

http://tyranny.egregious.net/~khrome/lego.The sketches and information there captured 

my imagination, and I had soon built several tri-star variations.To review some of my 

earlier designs, please take a look at: www.visi.com/~dc. 

While experimenting with these variations, it became clear to me that for stair 

climbing, as opposed to minor obstacle avoidance, it was necessary to provide separate 

drive mechanisms for both the wheel assembly and the wheels.The Stair-Climber model 

uses a differential to split the drive torque between the two separate drive modes. On a 

flat surface, the model will roll along like any normal wheeled vehicle. However, as soon 

as the model encounters enough resistance to start climbing, as when something blocks 

the wheel from rolling, the drive torque is transferred in order to rotate the wheel 

assembly that begins the climbing process. Each tri-star wheel has its own drive motor 

and differential to enable independent wheel action, as well as enough torque to easily 

climb a set of LEGO sized stairs at a reasonable speed. 

Another way to accomplish stair climbing would be to use separate drive motors for 

the climbing and rolling functions. Many sensors would be needed to determine the 

vehicle’s position and attitude relative to stairs and other terrain. If you had enough computing 

power and sensors you could possibly use only one pair of star-wheels, program


the robot to balance on two wheels, and climb stairs as well. Recently, Dean Kamen has 

created an incredible wheelchair (iBOT) with all these capabilities. It has two wheels per 

side used in a bi-star configuration (Figures 1.4 and Figure 1.5). Check out the following 

web sites to find out more on this incredible device: 

■ 

■ 

■ 

www.dekaresearch.com 

www.indetech.com 

www.dynopower.freeserve.co.uk/homepages/newchair.htm 

Figure 1.4 The iBOT Wheelchair in Elevated Mode 

Figure 1.5 iBOT Wheelchair Descending Stairs 

Building the Stair Climber 

There are two sets of instructions and two corresponding part lists for building the basic 

tri-star wheel assemblies: 

? The first option uses the older style TECHNIC tri-plate and associated toothed 

bushings. 

? The second option uses a newer tri-beam piece available in one of the LEGO 

Spybotic sets, and possibly other sets as well. 

As a third option, it would be possible to mix both types of tri-star wheels within the 

same model, as they are functionally equivalent. Use whatever combination you find convenient. 

When completed, the Stair-Climber is symmetrical from side to side and front to 

rear. Remember this when following the instructions, as many parts being added may be 

hidden from view.The only exception is the motor wiring, which is all tied to one 

common point.

6 Chapter 1 • Stair-Climber 

Engineering Trade-Offs 

So, by now you may be wondering where is the RCX? Well, as much fun as it would be 

to make this model autonomous by adding a few sensors and RCX, it really isn’t practical. 

All the extra weight is just too much for the tri-star assemblies of this model to 

function properly. When attempting to climb with higher loads, the excessive torque on 

the main tri-star axles leads to breaking axles and gears with higher loads. 

Beyond this, the main goal of this model was stair climbing and attaching an RCX to the 

model would raise the center of gravity (CG) enough to seriously limit vehicle stability, 

thus causing the model to flip over backwards when climbing steeper inclines. 

However, you could use an RCX as a handheld battery pack if you wanted to, and I 

will show you how this is possible toward the end of the chapter. But then again, the 

Stair-Climber doesn’t need the processing power, as it uses the differentials to shift 

between modes.There is one more reason for just using a battery pack with this particular 

model: if you try to turn this model by powering each side separately, you may find 

the wheels will slide out of place and jam. 

The Wheel Set 

The tri-star wheel set has a concentric drive arrangement to provide separate power for 

both rolling and stepping modes of operation.The differential housing used without 

internal gears provides this concentric drive mechanism and acts to hold the dark gray 

16T gears in place.You will need to build four of these.


Wheel Set Step 0 

Connect the #12 axle 

and the #2 axle together using 

the axle joiner as shown. 


Slide the bushings onto 

#12 axle as shown. 


Slide the tri-plate onto the #12 axle 

with orientation as shown. Install the 

three #4 axles and half-bushings so 

that the axle is aligned with its 

corresponding tri-plate section. 

There will need to be just enough 

axle extending behind the 

tri-plate to attach a halfthickness 

liftarm. 


Slide the bushings and a second triplate 

into place.



Attach the liftarms and axles as 

shown. Note that the three 

outer axles extend slightly from 

the rear of this assembly. This 

prevents the axles from interfering 

with the gears, 

added in the following 

step. 


Attach the gears, axles, 

and bushings as shown. 


Slide the pair of 

toothed bushings 

into place. 

Attach the wheels 

and check to 

make sure they 

rotate easily.



Slip the 16T gears into 

position with the smalltoothed 

section facing 

away from the wheels 


Slip the differential gear 

housing into place. 

Remember you 

will have to build 

four of these.


Building an Alternate Wheel Set 

If steering is deemed necessary, one could re-engineer the tri-star wheels by adding triplate 

support on both sides of the wheels instead of just a single drive side.The chassis 

would have to be modified to accommodate the wider wheel sets, and it would now 

make sense to use an RCX for a handheld power source.The control inputs could be 

either from an array of touch sensors or possibly a pair of rotation sensors configured as 

left and right joystick style inputs. A version of the Stair-Climber built with the Alternate 

Wheel Set sub-assembly would look like Figure 1.6. 

Figure 1.6 A Version of Stair-Climber Built with the 

Alternate Wheel-Set Sub-assembly


If you opt to build the Stair-Climber using the wider alternate wheel sets, you will 

need the following parts. 

This version is slightly stronger owing to the triple sandwich of the tri-beams. If you 

opt to build the Alternate Wheel Set sub-assembly, rather than the standard Wheel Set 

sub-assembly built earlier in the chapter, you will need to build four of these. 

NOTE 

Fear not, if you opt to build the Alternate Wheel Set sub-assembly, there are no 

changes to any of the other sub-assemblies in the Stair-Climber. The Frame subassemblies, 

and the Final sub-assemblies are compatible with either version of 

the Wheel Set sub-assemblies.


Alternate Wheel Set Step 0 

Connect the #12 axle and the #2 

axle together using an axle joiner 

as shown. 



Slide bushings onto #12 

axle as shown. 

Slide the tri-beams onto 

the #12 axles as shown. 


Insert axles as shown. There will 

need to be just enough axle 

extending behind the tri-beams to 

attach a half-thickness liftarm.



Attach the wheels and 

check to make sure that 

they rotate easily. 


Slide a half-bushing into place 

on main axle. 

Then slip the 16T gears into 

position with small-toothed 

section facing away from 

the wheels.



Slip the differential gear 

housing into place. 

Remember you will 

have to build 

four of these. 

The Mid-Frame 

The Mid-Frame sub-assembly is one of the components of the chassis that runs from the 

front to the rear of the model just inside the tri-star wheels.You will need to build two 

of these.


Mid-Frame Step 0 

Assemble parts as shown. 


Attach the beams and connector pins.



Attach an angled liftarm to combine 

the front and rear portions of this 

frame section. The half-bushings on 

either side of the beams are used to 

offset the frame. 


Attach the 1x2 bricks with axle 

holes to secure the structure. 

Remember you will have to build 

two of these.


The Outer-Frame 

The Outer-Frame sub-assembly is also a component of the chassis, similar to the Mid- 

Frame sub-assembly.The difference between the Mid-Frame sub-assembly and the Outer- 

Frame sub-assembly, is that the Outer-Frame sub-assembly is positioned on the outside of 

the tri-star wheels.You will also need to build two of these. 

Outer-Frame Step 0 

Insert the #4 axles into 1x2 bricks 

with axle holes. 


Attach an angled liftarm to 

combine the front and rear 

portions of the -frame 

section. Insert the full-length 

pins with stop bushings as 

shown.



Attach the 1x12 TECHNIC beams. 


Complete this structure by adding 

another angled liftarm as shown. 

Remember you will have to build two 

of these.


Putting It All Together 

Here is where we will complete the Stair-Climber. We will first build the central part of 

the model, and then attach the previous sub-assemblies in order.The directions show the 

original style tri-star wheel, but the assembly process is identical regardless of whether you 

opted to build the Wheel Set sub-assembly or the Alternate Wheel Set sub-assembly. 

When assembling the model, take care to be sure the parts are aligned exactly as shown. 

Because the model is symmetrical from side to side and front to rear, it should be easy to 

see if any parts are missing or misplaced. 

Final Step 0 

Insert the connector pins and axles 

into the angled liftarm. There should 

be equal lengths of axle extending 

out from either side of the liftarm


Final Step 1 

Attach a second 

angled liftarm and 

insert the connector 

pins. 

Final Step 2 

Attach the 1x2 bricks with 

axle holes and the 1x14 

TECHNIC bricks as shown. 

Final Step 3 

Attach a stack of two 1x4 plates 

and 1x4 TECHNIC brick on both 

ends of the structure.


Final Step 4 

Attach the beams as 

shown using the 

connector pins. 

Final Step 5 

Final Step 6 

Slide the #4 axles through the four 

24T gears and attach these to the 

beams as shown. Then attach the 

#10 axles.


Final Step 7 

Repeat the installation process 

performed in Final Step 6. The 

gears on each side should rotate 

freely without interference from its 

adjacent side. 

Final Step 8 

Attach the motors and bricks to each 

side of the chassis as shown.


Final Step 9 

Final Step 10 

Attach the parts as 

shown. This portion of 

the assembly is used to 

lock motors in place. 

First, attach an axle connector to 

each motor. Next, align each 

worm gear as shown, and pin in 

place with an axle. 

Final Step 11 

Rotate the model so 

that you are looking 

at the bottom side, 

and place four halfbushings 

as shown.


Final Step 12 

Final Step 13 

Attach the 

differential gear 

housings and bevel gears as 

shown. The last bevel gear for 

each housing will be added with 

the tri-star wheel assemblies in 

Final Steps 17 and 20. 

Attach parts as shown. 

Final Step 14 

Attach both Mid-Frame 

sub-assemblies as 

shown.


Final Step 15 

Attach plates as shown. 

Final Step 16 

Rotate the model to 

right side up, and attach 

the bricks and gears as 

shown.


Final Step 17 

In this step, you will add 

two of the Wheel Set 

sub-assemblies. 

In this view, you will note 

that the bevel gears are 

hidden. The bevel gears 

should be placed within 

the corresponding drive 

differential and held in 

place by the tri-star main 

axle. 

Final Step 18 

Locate an Outer-Frame subassembly 

and attach it as 

shown.


Final Step 19 

Attach the plates as shown.


Final Step 20 

Locate the remaining two 

Wheel Set subassemblies. 

These 

wheels are attached in 

the same manner as the 

Wheel Set subassemblies 

in Final 

Step 17.


Final Step 21 

Attach 

the second Outer- 

Frame sub-assembly 

and plates, similar to Final 

Steps 18 and 19.


Final Step 22 

Rotate the model as shown. Attach 

plates and electric wires as shown.


Final Step 23 

Attach two more electric wires.


Final Step 24 

Attach 

the 2x4 electric 

plate to the 

motor wires. The 

2x10 plates serve 

to hold the wires 

in place as well as 

to strengthen the 

assembly.


Final Step 25 

Attach a long electric wire between 

the 2X4 electric plate added in 

Final Step 25 and the 9V 

battery pack.


Operating the Stair-Climber 

Operating the Stair-Climber is relatively straightforward. Pushing the buttons on the battery 

pack should drive all four wheels forward or backward in unison. 

I suggest that you experiment and watch how the model drives, steps, and climbs over 

various obstacles. A pile of LEGO bricks is a perfect, re-configurable obstacle course.Try 

making stairs of various inclines using whatever is convenient. LEGO bricks work well 

for this, but books and scrap lumber are good alternatives as well. 

NOTE 

The CD-ROM that accompanies this book contains video of the Stair-Climber in 

action traversing various obstacles. 

Using an RCX instead of a Battery Pack 

Should you opt to use an RCX instead of the LEGO battery pack, your model might 

take the form of the Stair-Climber shown in Figure 1.7. 

Figure 1.7 Stair-Climber Built with an RCX instead of a Battery Box


It is a fairly simple process to modify your Stair-Climber so it is controllable by an 

RCX. First, you should attach a touch sensor to Input Port A, and second touch sensor to 

Input Port C, as shown in Figure 1.7.You will then have to modify the Stair-Climber 

motor wiring as shown.This change connects the left-side motors to Output Port 1 and 

the right-side motors to Output Port 3. 

NOTE 

The wires used for connecting the motors together need to be slightly longer 

than the ones supplied in the RIS 2.0 set. Use whatever combination of wires 

you have available to make these connections. 

Then, by writing a simple program in the language of your choice, assign the following 

values: 

■ 

■ 

■ 

Touch Sensor A causes both Output Ports 1 and 3 to be set to ‘reverse’ and ‘on’ 

while pressed. 

Touch Sensor C causes both Output Ports 1 and 3 to be set to ‘forward’ and ‘on’ 

while pressed. 

Both Output Ports 1 and 3 are turned ‘off ’ when neither Touch Sensor is 

pressed. 

A sample program built with the RIS 2.0 language and programming interface would 

look something like the program seen in Figure 1.8. 

Figure 1.8 A Sample Stair-Climber Program Built with RIS 2.0


Summary 

In this chapter, we have explored the use of a special type of star-wheel configuration 

designed specifically to overcome severe terrain obstacles including stairs.The model we 

built demonstrated some of the capabilities of this type of design. Others have used variations 

of the star-wheel for all terrain vehicles (ATVs) and wheelchairs. Future uses may 

include autonomous robots that have little difficulty navigating the same environments as 

we do. If ATVs are of a specific interest to you, I recommend that you jump ahead in the 

book to Chapter 6, and check out the Shape-Shifting Camera Tank built by 

Miguel Agulló.

Stair-Climber

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